Apparatus for performing a pull test

ABSTRACT

Apparatus for performing a pull test in order to determine the resistance of a bonded wire connection to tractional force, comprising a bonded-wire loop ( 5 ) between the two bonded contacts on a substrate ( 1 ), with a traction hook ( 7 ) that can be controllably inserted under the wire loop, a drive device that is connected to the hook and generates a tractional force directed substantially perpendicular to the substrate surface, a force-measurement device ( 21 ) associated with the hook to detect the tractional force at each moment, and a recording device connected to the force-measurement device in order to record a breaking-force value for the bonded-wire connection, wherein the force-measurement device ( 21 ) is disposed substantially coaxially with the tractional-force vector and detection means ( 40, 50, 90, 100 ) to detect the highest point of the bonded-wire loop are provided, as well as position control means ( 20, 30 ) for the automatic positioning of the traction hook, and hence the point of origin of the tractional-force vector, below the highest point of the bonded-wire loop.

DESCRIPTION

[0001] The invention relates to an apparatus for performing a so-called pull test, to determine tensile strength of a bonding wire connection tensile strength, according to the precharacterizing clause of Claim 1.

[0002] The pull test has long been the most commonly used method of testing bonding wire connections. In this method a small tension hook is inserted under a loop of wire between two bonding contacts and is moved away with constant velocity in a direction perpendicular to the surface of the substrate (i.e., usually upwards) until the loop breaks or a predetermined force has been reached. The tensile force applied at each moment is determined with a force-measurement device, and the value of tensile force that is being applied when the wire loop breaks away is recorded as the pull-off strength, which provides information about the quality of the bonding connection.

[0003] With this procedure, for instance, weakening of the wire in the heel region can be very well detected, as can the quality of the welding. The results of the test are compared with the minimal tensile strength for bonding wire connections specified in the relevant quality publications (e.g., MIL-STD883 method 2011).

[0004] Known test apparatus for carrying out this procedure, in which the force measurement is customarily accomplished by detecting the deformation the tensile force produces in an extension arm to which the hook is attached, have proved to be disadvantageous in certain respects. The precision of the measurements seems on the whole to be in need of improvement, and problems have arisen regarding reproducibility under conditions of changing ambient temperature.

[0005] The object of the invention is thus to make available an improved test apparatus of this generic kind, which in particular is designed to provide more accurate and reproducible test results even if the ambient temperature is variable.

[0006] This object is achieved by a test apparatus with the characteristics given in Claim 1.

[0007] The invention includes the essential idea of departing from the previously customary extension-arm principle and instead disposing the force-measurement device so that it is substantially coaxial with the tensile force vector generated by the drive unit. It further includes the idea of ensuring that the wire loop is reliably grasped at its highest point, so as to avoid errors in the measurement results introduced by force components deviating in direction from the perpendicular to the substrate surface. For this purpose, in accordance with the invention, detection means to detect the highest point in the bonding wire loop and position controllers for automatic positioning of the tension hook in such a way that the starting point of the tensile force vector is below the highest point of the wire loop are position are provided.

[0008] The force-measurement device preferably comprises at least one load cell so disposed as to be coaxial with a drive rod of the tension hook. In the interest of extensive temperature compensation, the force-measurement device preferably consists of a combination (a pair) of load cells disposed one above the other.

[0009] In another preferred embodiment an air bearing of the drive rod of the hook is provided, so as for practical purposes to eliminate the frictional forces that are produced in conventional bearings and that reduce the accuracy of the measurements.

[0010] In still another advantageous development of the idea behind the invention, a motor-gearbox unit is provided to rotate the tension hook about an axis aligned with the direction of action of the drive device, i.e. to fix the hook at a predetermined angle with respect to the tensile force vector. This measure serves on one hand to facilitate manipulation of the test apparatus in the case of substrates with complex bonding geometry, and on the other hand contributes towards the above-mentioned goal of increasing measurement accuracy, because it makes it possible to avoid potentially error-introducing positions of the hook relative to the wire loop.

[0011] Achievement of both of the above-mentioned advantages is also assisted by the provision of an x-y table for the coordinate-controlled positioning of the substrate (and hence of the highest point of the loop that the hook is meant to engage) with reference to the hook.

[0012] This measure should be regarded as closely linked to the provision of a camera with a field of view directed towards the side of the wire loop, and an image evaluation device connected to the camera, with which to calculate the coordinates of the highest point in the wire loop in the camera image. Alternatively, it can also be combined with the provision of a movement or proximity sensor associated with the drive unit, in particular the drive rod of the tension hook, and of a control and evaluation unit associated therewith. The latter serves to detect the maximal value of a plurality of positions at which the hook can engage the bonding-wire loop, and to determine the x-y coordinates of the engagement position at which the height is maximal. It will be clearly evident that the result of the measurement of the position of the highest point in the wire loop can be taken as a starting point for a correspondingly calculated shifting of the position of the sample table.

[0013] In another advantageous embodiment of the invention, spring means are disposed in association with the hook and the drive unit, in order to prestress the hook into a specified initial position, and also to attenuate the engagement between hook and wire loop and/or to limit the amount of tensile force, in order to protect the force-measurement device. In particular, the spring means comprise at least one first spring with low spring constant, for prestressing into the initial position and attenuate the engagement, and a second spring with high spring constant to limit the tensile force, both springs preferably being disposed coaxially with the drive rod. This arrangement results in a construction that is simple to manufacture and to maintain, while leverage and moments of tilt that might introduce errors into the measurement results are fundamentally ruled out.

[0014] Further features will be evident from the subordinate claims and the following description of an exemplary embodiment with reference to the figures, wherein

[0015]FIG. 1 is a schematic drawing of a wire loop to make clear the test principle,

[0016]FIG. 2 is another schematic drawing to illustrate the forces acting during a pull test,

[0017]FIG. 3 is a schematic diagram of a test apparatus according to one embodiment of the invention, and

[0018]FIG. 4 shows a longitudinal section of the main part of an embodiment of the test apparatus in accordance with the invention.

[0019]FIG. 1 shows schematically a bonding wire 5 bent into a loop between two bonding surfaces 1, 3. A tesnion hook 7 has been positioned under the wire to carry out the so-called pull test. The bonded wire 5 is attached to the bonding surfaces 1, 3 at contact points (bonding contacts) 5 a, 5 b. The reference numeral 5 c designates the highest point in the bonding wire loop. In the figure the hook 7 is in the optimal position for testing, namely under the highest point in the loop 5; for comparison, the dashed lines show a hook (not labelled with a number) that is displaced to the side and hence in a position unfavourable for testing.

[0020]FIG. 2 is a diagram illustrating the general case, in which the bond contacts are at different levels. It shows the angles and force vectors relevant to performing and evaluating the pull test, and the labels make it essentially self-explanatory. To make clear the geometric relationships between FIG. 1 and FIG. 2, in FIG. 2 the bond contacts 5 a, 5 b are identified; the components connected by the bonding wire (not shown in FIG. 2) in this case are a semiconductor circuit chip and a lead.

[0021] Regarding FIG. 2, in other respects reference is made to the above general discussion of the pull test.

[0022]FIG. 3 shows the basic construction of a test apparatus according to one embodiment of the invention in schematic diagram form; here it is being used to test the tensile strength of a wire loop 5 bonded to a substrate 1. The substrate 1 is fixed to an x-y table, movement of which is driven by a table drive unit controlled by a coordinate control stage 30.

[0023] A CCD camera 40, the output of which goes to an image evaluation device 50, is disposed at the side of the substrate 1 and records an image of the loop 5. (To simplify the drawing, the actual camera position is not shown correctly in the figure.) The output of the image evaluation device 50 is connected to an input of the coordinate control stage 30, so that it can signal to the latter the coordinates representing the position of the highest point in the loop 5, calculated on the basis of the camera image, and enable the x-y table 10 to be moved in accordance with the result of this evaluation.

[0024] The hook 7 is disposed at the lower end of a drive or measurement rod 13, which is supported in a practically frictionless manner in an air bearing 17 (this, like the drive rod, will be described in greater detail below). A motor-gearbox unit is provided to adjust the angular position of the hook. The drive or measurement rod 13 is attached to a force-measurement device 21 (which, likewise, is described in greater detail below with reference to FIG. 4), the output signal of which is sent to a recording unit 70. The test apparatus as a whole is driven by a measurement drive unit 80.

[0025] To the upper end of the drive rod 13 is attached a movement or proximity sensor 90 which is connected to a control and evaluation unit 100; the latter is additionally connected so that control and measurement signals can be sent from it to the measurement drive unit 80 and to the coordinate control stage 30. The control and evaluation unit, in cooperation with the coordinate control stage (and the table drive unit 20) as well as the measurement drive unit 80, serves to determine the position of the highest point on the basis of the signals from the movement sensor 90, which detects movements of the drive rod (measurement rod) 13 that follow the engagement of the hook 7 with the loop 5. In addition, it serves to report the coordinates of the highest point to the coordinate control stage 30 of the x-y table 10, in order to bring about a movement of the substrate 1 and hence the loop 5 such as to achieve the optimal position for engagement with the hook 7.

[0026]FIG. 4 shows a longitudinal section to illustrate essential parts of an embodiment of the test apparatus in accordance with the invention that is at present preferred. This is assembled on a mechanically stable supporting structure 9 that consists of several parts, and is protected by a cover 11. Evaluation devices and drive or power-supply components situated outside this structure are not shown in the figure; however, reference is made to these in the following description.

[0027] The tension hook 7 is attached to a drive rod 13, which simultaneously serves as a “measurement rod” to transmit the measurement force and is guided in a frictionless manner through an air bearing 17 in an air-bearing shaft 15. This arrangement ensures an especially low-friction supporting and hence a highly accurate measurement of the pull-off strength.

[0028] At the upper end of the air-bearing shaft 15 there is nonrotatably fixed to the drive rod 13 a cylinder gear 19, which meshes with the output pinion of an electric motor (neither of which is shown) and by way of which the hook 7 can be caused to rotate about the long axis of the drive rod 13, so as to change the orientation of the hook. As a result, a fine adjustment of the position of the hook with respect to the bonding connections to be tested is possible, even with difficult geometric constellations. This, too, thus contributes towards achieving a high measurement accuracy and cleanly reproducible results of the evaluation, even with bonding connections of a variety of configurations.

[0029] The drive or measurement rod 13 is surrounded coaxially by a force measurement device 21, which comprises two oppositely oriented load cells 23 a, 23 b of the conventional construction. Associated with the lower load cell 23 b is a so-called touchdown spring 25 for prestressing of the drive rod 13 and with it the tension hook 7, to fix the latter in the initial position. Associated with the upper load cell 23 a is an exchangeable main measurement spring 27, the spring constant of which is chosen to be appropriate for the measurement range that applies in each case. To guide the main measurement spring 27 and simultaneously to make available a first support surface for a protective spring (“Softweg” spring) 29 on the drive rod, a first spring sleeve 31 is provided. The upper part of the protective spring 29 is guided within a second spring sleeve 33, which in turn is supported in a ball bearing 35 for torsional protection of the above-mentioned springs.

[0030] Above the upper end of the drive or measurement rod 13 is disposed a fork photoelectric barrier 37 as a safety shut-off device for the test apparatus (in cooperation with evaluation electronics that are not shown).

[0031] The test apparatus described above is completed by means (not shown) for detecting and evaluating the position of the highest point in the wire loop—such as a CCD camera with suitable resolution, connected to a device to evaluate its output—and an x-y table by means of which a substrate bearing the wire loop to be tested can be moved with respect to the tension hook 7 in such a way that the latter can be inserted under the highest point of the wire loop, thus allowing an optimal accuracy and reproducibility of the test result to be achieved (cf. FIG. 3).

[0032] The embodiment of the invention is not restricted to this example and the aspects emphasized above, but is also possible in a large number of modifications that are within the competence of a person skilled in the art. 

1. Apparatus for performing a pull test in order to determine tensile strength of a bonding wire connection, comprising a bonding wire loop (5) between the two bonding contacts (5 a, 5 b) on a substrate (1, 3; CHIP, lead), with a tension hook (7) that can be controllably inserted under the wire loop, with a drive device that is connected to the hook and generates a tensile force directed substantially perpendicular to the substrate surface, with a force-measurement device (21) associated with the hook, to detect the tensile force at each moment, and with a recording device connected to the force-measurement device in order to record a pull-off strength value for the bonded-wire connection, wherein the force-measurement device (21) is disposed substantially coaxially with the tensile force vector (F), and detection means (40, 50, 90, 100) to detect the highest point (5 c) of the bonding wire loop are provided, as well as position control means (20, 30) for the automatic positioning of the tension hook, and hence the point of origin of the tensile force vector, below the highest point of the bonding wire loop.
 2. Apparatus according to claim 1, wherein the force-measurement device (21) comprises at least one load cell (23 a, 23 b) disposed coaxially with a drive rod of the tension hook.
 3. Apparatus according to claim 2, wherein the force-measurement device (21) comprises a pair of load cells (23 a, 23 b) disposed one above the other, coaxially with the drive rod of the tension hook.
 4. Apparatus according to claim 2, wherein the drive rod (13) of the tension hook is supported in an air bearing (15, 17).
 5. Apparatus according to one claim 1, wherein a motor-gearbox unit (60) to rotate the tension hook (7) about the direction of action of the drive device (80), i.e. to fix the tension hook in a predetermined angular position with respect to the tensile force vector (F) is provided.
 6. Apparatus according to claim 1, wherein the position control means comprises an x-y table (10, 20, 30) for the coordinate-controlled movement of the substrate, and hence the highest point of the bonding wire loop, with respect to the tension hook.
 7. Apparatus according to claim 1, wherein the detection means (40, 50) comprise a camera (40), the field of view of which is directed from the side onto the bonding wire loop (5), and an image-evaluation device (50) connected to the camera in order to calculate the coordinates of the highest point (5 c) of the bonding wire loop from the camera image.
 8. Apparatus according to claim 1, wherein the detection means (90, 100) comprise a movement and/or proximity sensor (90) associated with the drive device (80), in particular with the drive rod (13) of the tension hook (7), and connected therewith on the input side a control and evaluation unit (100) to detect the loop-height value of a plurality of positions in which the tension hook engages the bonding wire loop (5) and to determine the x-y coordinates of the engagement position (5 c) with the maximal height value.
 9. Apparatus according to claim 1, wherein with the tension hook (7) and the drive device (80) there are associated spring means (25-29) to prepress the tension hook in a specified initial position as well as to attenuate the engagement between tension hook and bonding wire loop and/or to limit the traction force in order to protect the force-measurement device (21).
 10. Apparatus according to claim 9, wherein the spring means (25-29) comprise a first spring (25, 27) with low spring constant for pretensioning into the initial position as well as for engagement attenuation, and a second spring (29) with high spring constant to limit the tensile force.
 11. Apparatus according to claim 10, wherein the first and second springs (25-29) are disposed so as to be coaxial with the drive rod of the tension hook. 